1. Field of the Invention
This invention pertains to a method for preparing flexographic printing plates and, in particular, a method for thermally treating a photosensitive element to form a relief structure suitable for flexographic printing.
2. Description of Related Art
Flexographic printing plates are well known for use in printing surfaces which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759 . The photo-polymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element. Upon imagewise exposure to actinic radiation, polymerization, and hence photocuring (insolublization) of the photo-polymerizable layer occurs in the exposed areas. Conventionally, the element is treated with a suitable solution, e.g., solvent or aqueous-based washout, to remove the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing.
However, developing systems that treat the element with a solution are time consuming since drying for extended period (0.5 to 24 hours) is necessary to remove entrained developer solution. In addition, these developing systems produce potentially toxic by-product wastes (both the solvent and any material carried off by the solvent) during the development process.
To avoid the problems with solution development, a “dry” thermal development process may be used. In a thermal development process, the photosensitive layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. Nos. 3,060,023 (Burg et al.); 3,264,103 (Cohen et al.); 5,015,556 (Martens); 5,175,072 (Martens); 5,215,859 (Martens); and 5,279,697 (Peterson et al.). The exposed portions of the photosensitive layer remain hard, that is do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated/removed from the photosensitive layer. The cycle of heating and contacting the photosensitive layer may need to be repeated several times in order to sufficiently remove the flowable composition from the un-irradiated areas and form a relief structure suitable for printing. Thus remains a raised relief structure of irradiated, hardened composition that represents the irradiated image.
The photosensitive compositions suitable for use in thermal development process as disclosed by Martens in the above citations primarily are radiation curable polyurethane elastomers. Martens discloses that the polyurethane elastomer have a melt transition temperature of less than 180° C. and a melt index of at least 0.5 grams/minute at 180° C., and, that the melt indices of molten radiation crosslinkable elastomeric compositions when measured according to ASTM No. D 1238-70 should be in the range of 0.5 grams/minute to 10.0 grams/minute in the temperature range from 100° C. to 180° C.
In U.S. Pat. No. 5,215,859 Martens also discloses that commercially available flexographic printing plates, e.g., DuPont PLS, can be used in a dry development process for preparing printing plates. (Commercially available plates, such as PLS are prepared using conventional solution, e.g., solvent, washout processes to form the relief.) However, a PLS plate had poor clean out, i.e., removal of polymer, of the uncured portions, i.e., poor relief depth, upon thermal development and also distortion of fine printing elements and highlight dots could be seen. In order to improve the removal of the polymer (clean out) of the uncured portions without extending the development time, the development temperature was raised. But raising of the development temperature caused even more severe distortion of the fine image elements of the plate and/or polymeric base support for the plate.
So a problem arises with the use of conventional and/or prior art flexographic printing elements in a thermal development process for preparing printing plates in that clean-out of uncured portions is insufficient to form a relief suitable for quality printing of highlight dots and fine lines. Efforts to improve the clean out of these elements are counter to the desired performance of the system and plate. Increasing number of cycles of heating and contacting of the photosensitive layer is counter to maintaining the productivity of the thermal development system. Increasing the temperature to heat the photosensitive element and to cause the uncured portions of the layer to melt or flow more readily may be contrary to maintaining dimensional integrity of the element.